a) Field of the Invention
The present invention relates to an electrode capable of achieving a good cell performance, and more particularly to an electrode having good repetitive charge/discharge characteristics and to a non-aqueous electrolyte secondary cell using such an electrode.
b) Description of the Related Art
Secondary cells are widely used as power sources of electronic apparatuses of high quality and compact portable size. Of these secondary cells, recently developed lithium cells have a large capacity and a large output so that demands for these cells as power sources of portable electronic apparatuses are rapidly expanding. It is known, however, that the capacity of a secondary cell gradually lowers after charge and discharge are repeated.
The invention has been made in consideration of such circumstances and is based upon the finding that the tip portion of an electrode of the secondary cell has a high potential and because of this the capacity of the cell gradually lowers after repetitive charge/discharge cycles.
The electric conductivity of non-aqueous electrolyte used by a non-aqueous secondary cell is lower than that of aqueous or water-containing electrolyte. It is therefore necessary to broaden an electrode area. From this reason, a usual cylindrical non-aqueous secondary cell uses an electrode sheet which is wound in a cylindrical cell can. This electrode sheet has a thin film coated on one or both surfaces of a conductive support sheet, the thin film being made of a depolarizing mix (conductive materials of positive electrode active material, negative electrode material, conductivity-imparting material, binding material, and the like).
The electrode sheet is required to be electrically connected via a positive or negative electrode lead plate to a positive or negative electrode terminal of a cell so that a partial area of the conductive support sheet (usually one or both ends thereof) must be exposed for such electrical connection. In a usual electrode sheet, an exposed area not covered with the depolarizing mix layer is therefore formed to expose the conductive sheet.
A number of non-aqueous secondary cells are required to be mass produced with low cost in order to follow a recent expansion of cell application fields. To meet such requirements, electrode sheets are generally manufactured by coating a depolarizing mix layer of electrode materials on the surface of a stripe conductive sheet and thereafter cutting the sheets into separate electrode sheet pieces having a predetermined length. Even with such a electrode sheet manufacture method using the stripe conductive sheet and cutting it, it is obvious that the exposed area is required to be formed on the conductive sheet. Conventionally, the depolarizing mix layer is peeled off from the end portion of a cut electrode sheet. However, a work efficiency of this method is very bad and various other methods have been proposed.
According to a method described in JP-B-SHOU-55-31989, an adhesive tape is bonded to a partial area of a conductive sheet piece and a depolarizing mix layer is formed on the conductive sheet piece. Thereafter, the adhesive tape is peeled off to thereby form a partial area of the electrode sheet not covered with the depolarizing mix layer (an exposed area of the surface of the conductive sheet). This method is an improved method which mechanically peels off the depolarizing mix layer from the electrode sheet. However, this method is applied to each separate electrode sheet piece so that it is not suitable for mass production of electrode sheets.
According to JP-A-HEI-1-184069 and JP-A-HEI-1-194265, a doctor blade method is disclosed which has a shutter capable of controlling to intermittently supply coating liquid for forming an electrode depolarizing mix layer. Namely, with this doctor blade method, a stripe conductive sheet is coated with depolarizing mix layer pieces at an interval which leaves the non-coated areas between adjacent depolarizing mix layer pieces. These areas of the conductive sheet not coated with the depolarizing mix layer are used for electrical connection to positive or negative electrode plates.
This method is supposed to be theoretically suitable for mass production of electrode sheets. However, according to the teachings disclosed in JP-A-HEI-8-45501, boundaries between the areas of the electrode sheet covered with the depolarizing mix layer and the areas not covered with this layer become likely to be irregular, and in addition to this the thickness of the depolarizing mix layer becomes likely to be irregular.
This JP-A-HEI-8-45501 also discloses a method of coating a stripe conductive sheet with depolarizing mix layers at an interval which leaves the non-coated areas between adjacent depolarizing mix layers. In this method, liquid for forming a depolarizing mix layer on the surface of a stripe conductive sheet at a predetermined interval is supplied with an extrusion type injector having a slot nozzle. It is described in JP-A-HEI-8-45501 that use of such an injector can suppress irregularity of boundaries between the area covered with the depolarizing mix layer and the area not covered therewith and improve the uniformity of thickness of the depolarizing mix layer.